Gibilisco S. Meteorology Demystified

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7. If the main vortex of a tornado rotates at 120 kt, there are secondary vor-

tices rotating at 40 kt, there are suction vortices rotating at 50 kt, and the

whole system is moving at 20 kt, with respect to the surface from

the southwest towards the northeast, the highest winds blow at

(a) 120 kt.

(b) 160 kt.

(d) 230 kt.

8. An approaching tornado can often, but not always, be detected by

(a) the image of a mesocyclone on a radar set.

(b) the brightening of a television screen tuned to a vacant channel.

(d) Any of the above

9. On a spring or summer day in an American Midwestern location,

you should be aware of a greater-than-normal chance for tornadoes to

occur if

(a) the barometer is falling and the wind is blowing from the south or

southeast.

(b) severe thunderstorms appear to be developing.

(d) Any of the above

10. A tornado warning will be issued when

(a) conditions are favorable for tornado development.

(b) a funnel cloud has been sighted.

(d) the barometer is falling.

CHAPTER 5 Tornadoes

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CHAPTER

159

Tropical Cyclones

Hurricanes are the most widely publicized of all storms. They occur every year

in both the northern and southern hemispheres. These storms form over the

warm waters of oceans in the tropics and subtropics, but they often end up in the

temperate regions. In this chapter, we will see how tropical cyclones form,

where they occur, what they do, and how people can minimize the dangers posed

by tropical cyclones.

A Hypothetical Storm

The date: September 10. The time: 6:20 P.M. Eastern Standard. The weather fore-

cast contains a statement that a tropical depression is under scrutiny. It is located

in the far eastern Atlantic and shows signs of strengthening. By 11:00

P.M. that

day, the depression has become a tropical storm with maximum sustained winds

(based on a 1-min average) of 34 kt (39 mi/h). It is the fourth tropical storm of

the year in the North Atlantic, and is given the name Debby. The following

morning, Debby is rapidly gaining strength. The center of circulation is speci-

fied as 11.5°N and 40.4°W, far out in the open sea. Debby is, at the moment, a

threat only to shipping interests.

In the North Atlantic, the eastern North Pacific, and the central North Pacific

oceans, intense tropical cyclones are known as hurricanes. In the western North

Pacific, they are called typhoons. Severe typhoons are called supertyphoons. In

the Indian Ocean, tropical storms and hurricanes are called cyclones. In Australia,

hurricanes are sometimes called gales.

STRENGTHENING

By the evening of September 11, Debby packs maximum sustained winds of

64 kt (74 mi/h), and has thus become a minimal hurricane. Conditions favor

further strengthening. The storm is moving almost directly westward at 18 kt

(21 mi/h). The television newscasters show a satellite photograph of Debby. The

storm appears as a pinwheel of clouds with a small black dot near the center:

a defined eye. We will now keep ourselves informed of the progress of Debby,

as will millions of residents of the eastern and Gulf coasts of the United States.

Atlantic hurricanes pose a perennial threat to the United States, as well as

Mexico and much of Central America. The eye of Debby, at its present low

latitude, moves in an almost westward direction, but it is almost certain that

the storm will eventually recurve toward the northwest. By the morning of

September 12, Debby has the full attention of the National Hurricane Center.

She has strengthened with phenomenal rapidity, reminiscent of Hurricane Allen

in 1980. The maximum sustained winds are estimated at 150 kt (about 175 mi/h)

with gusts to 175 kt (about 200 mi/h). On a scale of intensity from 1 to 5, Debby

is a strong 5.

By the evening of September 12, the eye of Debby is located near 12.0°N and

52.9°W. The islands of Grenada, St. Vincent, St. Lucia, and Barbados await the

arrival of Debby’s fury as seas reach 20 m (66 ft) and breakers pound with unre-

lenting violence on the windward beaches. The normally gentle trade winds shift

to the north, and acquire an ominous gustiness. Then, one after the other, the

islands are cut off from civilization.

HORROR STORIES

The first reports from Barbados and nearby islands trickle out from emergency-

powered amateur (“ham”) radio stations, a mode of communication that always

continues to function when conventional modes fail. Almost every building on

these islands has been flattened or gutted. There are reports of human casualties.

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The hurricane track begins to recurve. The eyewall, or most destructive part

of the hurricane, is about 160 km (100 mi) in diameter. St. Lucia gets a brief rush

of high winds and heavy rain, but escapes the brunt of the storm. The hurricane

has slowed its forward motion to 13 kt (15 mi/h) as she lumbers into the

Caribbean, heading in the general direction of Haiti and the Dominican

Republic. Further reports come from Barbados, which received a direct hit.

Trees have been stripped of their bark and leaves; the landscape is barren and

desolate; it looks as if there has been a nuclear war. Homeless people wander

through the debris.

The fate of the Gulf coast and eastern seaboard of the United States now

depends on a number of factors. Atmospheric steering currents, which direct

hurricane paths, are difficult to predict. Computers at the National Hurricane

Center in Miami, Florida, work around the clock, continually updating the

status and position of Debby. Sustained winds are near 150 kt (about 175 mi/h).

The eye is about 24 km (15 mi) across. Hurricane-force winds cover a roughly

circular area with a diameter of 240 km (150 mi).

The hurricane slams into the island of Hispaniola with sustained winds so

violent that all the measuring apparatus is destroyed. Residents of Florida pre-

pare for a potential onslaught. Everyone on the Gulf coast also watches the

progress of the storm. The high hills of Hispaniola weaken Debby. As she

emerges into the northern Caribbean, her intensity has diminished. Weather fore-

casters know this is typical.

FLUID FORCE

The strongest winds in a hurricane are difficult to measure because instruments

rarely survive. The device generally used to measure wind speed is the

anemometer (Fig. 6-1). This device spins as the wind is caught in the concave

parts of the cups. The greater the wind speed, the faster the device rotates. The

rotating shaft is connected to an electric generator or tachometer that measures

the angular speed and translates this into knots. If the wind is too strong, the

assembly can be damaged or destroyed. The highest sustained winds in the most

intense hurricanes are thought to be around 175 kt (200 mi/h), with gusts

approximately 25 to 30 kt (30 to 35 mi/h) higher than that.

The force produced by the wind in an intense hurricane such as Allen (1980),

Gilbert (1988), or Andrew (1992) is difficult to imagine for people who have not

been through the experience. In a typical severe thunderstorm in the midwestern

or eastern United States, sustained winds of minimal hurricane force (64 kt or

CHAPTER 6 Tropical Cyclones

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74 mi/h) sometimes occur. In such storms, trees are uprooted, windows are

blown out, some roofs are damaged, utility wires are blown down, small aircraft

are overturned, and a few automobiles are wrecked. Wind speeds in the most

intense hurricanes are a little more than twice this, but the resulting damage is

many times greater. The damaging effects of high winds increase much more

rapidly than the numerical wind speed.

The most intense part of a hurricane occurs over a rather small area, usually

less than 300 km (185 mi) in diameter. The forward progress of a typical hurri-

cane is only about 10 to 15 kt (12 to 17 mi/h). Therefore, even a fast-moving

hurricane will subject some places to high winds for several hours. A large hur-

ricane, if it is slow-moving, can pound a single surface location with high winds

for more than 24 h.

WHERE WILL DEBBY GO?

Common sense suggests that there ought to be some way to foretell, at least

approximately, where our hypothetical storm called Debby will go, whether it

will make landfall on the continent, and if so, where. The art of hurricane-path

forecasting is improving continually, but there is still plenty of uncertainty.

CHAPTER 6 Tropical Cyclones

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Wind

Fig. 6-1. An anemometer cup assembly rotates as the wind blows past. The rate of

rotation (angular speed) of the device is proportional to the wind speed.

Hurricanes that originate in the far eastern Atlantic can eventually strike the

coast of the Western world anywhere from Honduras to Newfoundland; many

miss North America altogether and dissipate in the chilly waters of the far

North Atlantic.

The path that a hurricane takes depends on many factors. One significant

influence on tropical storm tracks can be inferred from the climate-control func-

tion they serve. The tropics receive more heat from the sun than they radiate into

space. The polar regions radiate more heat than they get from the sun. The dif-

ference must be made up somehow, in order to prevent a runaway thermal

imbalance in which the equatorial regions of our planet would become like

Venus and the poles would become like Mars! Hurricanes are a major vehicle for

the transfer of thermal energy from the tropics toward the poles. This is why

many hurricanes tend to recurve northward in the northern hemisphere, and

southward in the southern hemisphere.

Meteorologists are constantly working to improve their methods of tracking

hurricanes and predicting their paths. The same basic principles apply to all hur-

ricane regions in the world. The arrangement of continental weather systems, the

presence of land masses, and the temperature of the ocean each play a crucial

role in the ultimate track that a hurricane will follow. These variables also deter-

mine where a storm will form, how it will evolve, how long it will last, and how

it will ultimately dissipate.

PROBLEM 6-1

Can North Atlantic hurricanes ever miss the Americas, recurve into the

far North Atlantic, and then strike Europe or England?

SOLUTION 6-1

This is rare, but it has happened. Autumn gales that occur in the rural

western part of England can sometimes be traced back to tropical origins.

The Azores-Bermuda High

At all times of the year, but especially during the summer in the northern hemi-

sphere, the prevailing winds over the North Atlantic revolve clockwise around a

persistent region of high pressure. Because this system is usually centered near

the Azores or the island of Bermuda, it is often called the Azores-Bermuda high,

or simply the Bermuda high. This system is responsible for much of the weather

CHAPTER 6 Tropical Cyclones

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that occurs along the east coast of the United States in the summer. In other parts

of the world, similar tropical or subtropical high-pressure systems exist over the

oceans. In the northern hemisphere, the circulation around these systems is in a

clockwise direction; in the southern hemisphere it is counterclockwise.

CONVERGENCE OF THE EASTERLIES

Near the equator, the belts of tropical easterly winds meet. This is the intertrop-

ical convergence zone (ITCZ), which was introduced in Chapter 2. It is a region

of low pressure and light winds. Thundershowers often form in this zone. In

ancient times, this region was called the doldrums, because sailing ships passing

through it on their way from the northern hemisphere to the southern, or from

the southern hemisphere to the northern, were often becalmed for lack of wind.

In the Atlantic, the ITCZ almost always lies north of the equator. In August and

September, the ITCZ reaches its northernmost position.

Hurricanes can develop when the ITCZ gets far enough from the equator to

allow significant Coriolis rotation. In the northern hemisphere, this rotational

force takes place in a counterclockwise direction. In the southern hemisphere, it

occurs in a clockwise direction. Because the ITCZ is almost always north of the

equator in the Atlantic, hurricanes are rarely observed in the South Atlantic. (In

March of 2004, the first South Atlantic tropical cyclone on record occurred. It

struck Brazil and caused considerable damage. Some scientists suggest that global

warming may result in more frequent South Atlantic cyclones in the future.)

WAVES, DEPRESSIONS, AND STORMS

All hurricanes begin as tropical waves. These are first observed as irregularities,

or “bumps,” in the tropical isobars (Fig. 6-2). Showers and thunderstorms

develop on the eastern, or trailing, side of the disturbance. Tropical waves, also

called easterly waves, are common during the summer months, and they usually

move from east to west without intensifying. But when conditions are favorable

for intensification, an easterly wave can develop a cyclonic circulation. When

some of the isobars become closed curves, indicating that a cyclonic circulation

has developed, the wave becomes a tropical depression.

A tropical depression is an area of low pressure, similar to a temperate low

but without cold fronts or warm fronts. Such a disturbance can continue to

strengthen. When this occurs, the central pressure keeps falling, and the wind

speed increases. Warm air near the center tends to rise, and the surface winds

CHAPTER 6 Tropical Cyclones

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spiral inward. When the maximum sustained wind reaches 34 kt (39 mi/h), the

disturbance becomes a tropical storm, and meteorologists give it a name. If

intensification continues until the sustained wind reaches 64 kt (74 mi/h), the

storm officially becomes a hurricane.

HURRICANE “BREEDING ZONES”

Hurricanes form just north or south of the equator in many parts of the world

(Fig. 6-3). At the equator, there is no Coriolis force, and the big whirlwinds can-

not develop; the latitude must normally be greater than 5° either north or south.

Hurricanes almost always mature in the tropics, but a few mature in the temper-

ate zones. The key is ocean temperature, which must be above approximately

27°C (80°F) in order for a storm to reach hurricane strength. Hurricanes that

threaten the eastern United States mature in the Atlantic Ocean, the Caribbean

Sea, or the Gulf of Mexico.

In the northern hemisphere, the hurricane season begins on June 1 and ends

on November 30. In the southern hemisphere, the season is reversed. In some

parts of the world, hurricane seasons occur at other times; a good example is the

Bay of Bengal, where storms occur frequently in April and again in October. The

CHAPTER 6 Tropical Cyclones

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Fig. 6-2. A tropical wave appears as a “bump” in the isobars on the equatorial side

of an oceanic high-pressure system.

greatest frequency of hurricanes in most of the northern hemisphere is in the

months of August and September. This is also the time when a severe storm,

such as our hypothetical Hurricane Debby, is most likely to develop.

At the beginning of the Atlantic hurricane season, storms usually mature in

the southwestern part of the Caribbean Sea and in the Gulf of Mexico. By late

July, the temperature of tropical Atlantic waters has warmed up sufficiently to

allow storms to intensify in that region. By mid-October, the main area of devel-

opment returns to the southwestern Caribbean and the Gulf of Mexico.

In some years, several hurricanes form in the Atlantic, the Caribbean, and the

Gulf of Mexico. In other years, there may be few or no storms that reach hurri-

cane intensity in that region. Meteorologists have found some correlation

between unusual warming of the equatorial Pacific off the coast of South

America (the El Niño phenomenon) and a lack of hurricane development in the

Atlantic and Caribbean. However, El Niño events seem to cause an increase in

the frequency and severity of hurricanes in the western Pacific.

Once a hurricane has formed, it has a natural tendency to move away from

the equator, although this does not always happen. The precise path of a partic-

ular storm depends on weather conditions over the continents and the locations

of the high and low pressure systems at the higher latitudes. Sometimes severe

storms actually turn toward the equator before dissipating.

PROBLEM 6-2

Give an example of a major hurricane that turned toward the equator

before doing most of its damage.

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Fig. 6-3. Hurricane “breeding zones” and common tracks throughout the world.

SOLUTION 6-2

Hurricane Mitch of 1998 intensified over the Caribbean and then turned

south, spending most of his energy over Honduras. After weakening,

he moved north and northeast causing some wind and rain in South

Florida, and finally dissipated over the North Atlantic.

Anatomy of a Hurricane

Although hurricanes are low-pressure systems, they differ from the lows of the

temperate latitudes. The hurricane is more symmetrical than the cyclonic storms

familiar to inhabitants of Europe, the interior United States, and other temperate

regions. The hurricane, in its tropical stage, does not contain frontal systems.

The isobars are almost perfect circles, especially in a well-developed storm

(Fig. 6-4). The central pressure is usually lower (and sometimes much lower)

than that of a cyclone in the temperate regions. When a hurricane contains nearly

circular isobars near its center, it is said to have tropical characteristics. As

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Fig. 6-4. Isobars around a hypothetical hurricane near the Gulf Coast of Florida.

Barometric pressures are in inches of mercury (inHg), a unit commonly

used in weather broadcasts to the general public in the United States.